Interfacial reaction and solder joint reliability of Pb-free solders in lead frame chip scale packages (LF-CSP)

Chip scale packages (CSP) have essential solder joint quality problems, and a board level reliability is a key issue in design and development of the CSP type packages. There has been an effort to eliminate Pb from solder due to its toxicology. To evaluate the various solder balls in CSP package app lications, Pb-free Sn-Ag-X (X = In, Cu, Bi) and Sn-9Zn-1Bi-5In solder balls were characterized by melting behavior, phases, interfacial reaction, and solder joint reliability. For studying joint strength between solders and u nder bump metallurgy (UBM) systems, various UBMs were prepared by electropl ating and electroless plating. After T/C (temperature cycle) test, Sn3.5Ag8 .5In solder was partially corroded and its shape was distorted. This phenom enon was observed in a Sn3Ag10In1Cu solder system, too. Their fractured sur face, microstructure of solder joint interface, and of bulk solder ball wer e examined and analyzed by optical microscopy, SEM and EDX. To simulate the real surface mounting condition and evaluate the solder joint reliability on board level, Daisy chain test samples using LF-CSP packages were prepare d with various Pb-free solders, then a temperature cycle test (-65 similar to 150 degrees C) was performed. All tested Pb-free solders showed better b oard level solder joint reliability than Sn-36Pb-2Ag. Sn-3.5Ag-0.7Cu and Sn -9Zn-1Bi-5In solders showed 35%, 100% superior solder joint reliability tha n Sn-36Pb-2Ag solder ball, respectively.